Partial decarburization of iron and steel



Jan. 16, 1968 W. H. MlNCK PARTIAL DECARBURIZATION OF IRON AND STEEL Original Filed Sept. 21, 1964 INVENTOR. WILLIAM H. MINCK,

BY 1m i-uw,

ATTORNEYS" United States l atent O 3,364,983 PARTIAL DECARBURIZATION OF IRON AND STEEL William H Minck, Miamisburg, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Continuation oi application Ser. No. 397,982, Sept. 21, 1964. This application May 10, 1967, Ser. No. 641,737

3 Claims. (Cl. 148-16) ABSTRACT OF THE DISCLOSURE Process for the uniform partial decarburization of iron and steel strip, the strip in loose coil form being heated in a furnace in a non-oxidizing atmosphere until all parts of the strip are at a uniform temperature, whereupon a decarburizing atmosphere is introduced into the furnace and maintained until the strip has been decarburized to a specified intermediate carbon level, decarburization being terminated when the specified carbon level is reached, and the partially decarburized strip thereafter being cooled in a non-oxidizing atmosphere.

This is a continuation of copending applcation Ser. No. 397,982, filed Sept. 21, 1964, and entitled, Partial Decarburization of Iron and Steel, now abandoned.

Background of the invention It has been known since the United States Patent No. 2,287,467 in the name of Carpenter and Jackson, that iron or steel strip stock of sheet thickness could be rapidly decarburized in a continuous annealing furnace at a relatively low temperature through the use of a wet-reducing atmosphere. The temperatures employed were generally in the range of 1300 F. to 1600 F.; and it was not difficult to reduce the carbon content of ferrous material initially containing up to or above about 0.1% carbon to a value at least as low as about .003%.

Decarburization in a wet reducing atmosphere will hereinafter be termed gaseous decarburization for convenience. It is a procedure in which the metal is heated in an atmosphere oxidizing to carbon but reducing to iron. Oxidation of other elements may also occur, e.g., silicon, if the ferrous material contains any appreciable amount of it, as in silicon-iron for magnetic purposes. As the carbon at the surfaces of the strip stock oxidizes, passing oil in gaseous form principally as carbon monoxide, carbon from the interior of the stock migrates to the surface so that a uniform decarburization of the stock throughout its thickness can be achieved.

However, the heat treatment involved in the type of gaseous decarburization which has been described, is not always the most desirable one for the development of the physical or electrical quantities of the stock. There are many instances in which a longer heat treatment such as is characteristic of a box anneal would be highly desirable. Gaseous decarburization is not as easily carried on in a box anneal because it requires free access of the decarburizing atmosphere to all surfaces of the ferrous material. If stacked sheets or tightly Wound coils are used, the provision of an annealing separator on the surfaces of the stock is necessary; and this interferes with the aforesaid free access. Moreover, in box annealing procedures, stacked sheets and tightly wound coils present the equivalent of a large mass of metal irrespective of the gauge of the sheet stock. It is difficult and time-consuming to heat all parts of the mass to the same temperature, so that overheating of edge and surface portions is likely to occur with the production of burned areas or areas of the metal in which substantially more grain growth has occurred than in other portions of the stock.

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Some recent procedures have been developed in which a coil or coils of the metal are heated upon stands in a rn-uflle furnace, the coil or coils being loose i.e., in a condition in which the convolutions are separated sufficiently to permit access of the decarburizing atmosphere to all surfaces of the metal being treated. The provision of loose coils is usually accomplished by coiling the metal with a strand element to separate the convolutions. In some instances a strand of nylon or other suitable substance is used, and then removed from the coil before the annealing. In other instances, a metallic strand so configured as to permit passage of the gases is wound into the coil and is left in place during the annealing. Many of the furnaces designed to anneal loose coils are provided with fans to enforce circulation of the annealing atmosphere, and in particular to drive the atmosphere through the coils between the convolutions thereof. The coils are placed on end. The muffle is provided with means for the entrance and exit of the atmosphere so that the atmosphere can be changed, or so that it can be controlled by treating it outside the mufile and recirculating it.

The loose coil decarburization of iron and steel strip materials has proved successful. It is normally carried out by introducing the wet decarburizing atmosphere into the mufile as soon as the top of the coil (which is the hot test part of the coil before equilibrium temperature conditions have been attained) reaches a decarburizing temperature of about 1300 F. At this time, the coldest part of the coil will be at a temperature of about 1100 P. which is hot enough to prevent oxidation of the steel. The decarburizing atmosphere which in this instance will be a wet reducing gas, is continuously passed through the muflle as the cooler portions of the coil or coils come up to decarburizing temperature. The temperatures of various parts of the coils can be determined by thermocouples or similar means. The decarburizing process has hitherto been considered complete when the carbon monoxide level in the exit gases drops to about 1.0% or less, e.g., .7%. This indicates that the decarburizing reaction has slowed down to the point where continued treatment does not produce a proportionate lowering of residual carbon. The product has been found to be uniformly decarburized and to have a carbon content of about .002% or somewhat lower.

In the manufacture of iron and steel sheet stocks, it is not always necessary to achieve so low a carbon content; and for many purposes a higher carbon content is desirable. There has not hitherto been any satisfactory way of achieving controlled decarburization to intermedii ate values (hereinafter called partial decarburization). Since the carbon monoxide content of the gas passing out of the furnace has been utilized as an indication of carbon removal, it would ordinarily be supposed that higher carbon values in the metal being treated would result if the decarburizing action were stopped at a higher carbon monoxide value in the exiting gas. Cessation of the decarburizing action can easily be accomplished by changing the atmosphere to a non-decarburizing gas. Attempts to do this, however, were unsuccessful. Instead of having a uniformly higher resident carbon content, the coils were found to have widely varying carbon contents. In general, the decarburizing reaction tended to go to completion at the top of the charge, While very little, if any, decarburization occurred near the bottom of the charge. Originally this was thought to be due to a depletion of the decarburizing potential of the furnace atmosphere by the first portions of the ferrous material with which it came into contact, so that by the time the gases reached the bottom of the charge they had become, to all intents and purposes, inactive for decarburization. But this was found by later work not to be true; and in particular the use of furnace atmospheres having a higher decarburizing potential was not found to be a solution of the problem.

Summary of the invention The basic object of the invention is to provide a meth-- od whereby in the open coil annealing of iron and steef strip stocks the decarburization can be stopped at any desired carbon level, while at the same time the stocks. will be found uniformly decarburized to the selected level. So far as applicant is aware, this has not hitherto been. attained in this particular art. The present method con-- templates the heating of strip having an initial carbon content in excess of .0l% carbon in a cirlulating nonoxidizing atmosphere in a mutfle furnace to a selected uniform temperature in a range of from llOO F. to 1600 F., introducing water vapor into the atmosphere to raise the dew point to a value between 20 F. to 120 F. when the strip has reached the selected uniform temperature, thereby providing a decarburizing atmosphere which is maintained until the strip has been decarburized to a specified intermediate carbon level of at least .005% and greater than the complete decarburization level for the particular strip material being treated. When such specified intermediate carbon level is reached, which may be determined by monitoring the carbon monoxide level of the atmosphere removed from the furnace, the introduction of Water vapor is discontinued and the annealing cycle completed in a non-oxidizing atmosphere. The annealing atmosphere introduced into the furnace is free from carburizing gases and contains sufficient hydrogen to prevent oxidation of the strip at the temperatures involved.

The drawing The drawing is a curve showing how uncombined carbon in the material being treated will vary with the percentage of carbon monoxide in the decarburizing atmosphere.

Description of the preferred embodiment It should be emphasized that this invention relates to partial decarburization; i.e., if decarburizing were continued, the percentage of carbon in the stock would be further reduced. In ordinary killed or rimmed steels having initially about .04% carbon, complete decarburization would leave about .002% carbon. By the teachings of this invention, decarburization of this kind of material can be terminated at a uniform value of .005% carbon or higher.

The skilled worker will understand that the level of complete decarburization will vary with composition. Some mild steels are produced with small amounts of alloying elements to increase strength, refine the grain size, etc. These elements which are strong carbide formers (Ti, V, Cb, and the like) will form carbides and render the carbon unavailable for removal. As an example, a steel containing .06% titanium would be completely decarburized when the carbon reaches about .01%. This much carbon could be combined with the titanium as a stable carbide and is not removed through ordinary decarburization.

The teachings of this invention will work on any of these materials but, of course, the carbon content of the partially decarburized product will be dependent upon the quantity of carbide formers present in the steel.

In the practice of the invention, it has been found that non-uniform decarburization is primarily due to a thermal gradient. Even where the metal is in the form of loose coils, it requires a substantial length of time to bring all parts of the coil uniformly to the selected decarburizing temperature. But if the furnace charge is first heated in a non-decarburizing atmosphere to a uniform decarburizing temperature, and if the flow of the wet decarburizing atmosphere is not started until uniformity of temperature is attained, decarburizalion will surprisingly proceed at a uniform rate on all portions of the charge despite the fact that the atmosphere has to pass certain portions of the metal to reach other portions. Thus it becomes possible to stop the decarburization when a desired level of residual carbon has been reached, and still obtain a uniform product. At temperatures of the order of about 1100 to about 1600 F, the amount of moisture in the atmosphere will determine whether or not its action will be decarburizing; and an annealing atmosphere is generally made Wet by introducing into it a controlled quantity of steam before the atmosphere enters the mufile. A. decarburizing atmosphere may therefore be simply changed to a non-decarburizing atmosphere in most instances by shutting 01f the flow of the steam. Reference may be made to US. Patent No. 3,127,289 in the name of F. W. Beall, which talks of various controls for annealing atmospheres.

At the temperatures involved, decarburization in a Wet atmosphere proceeds in accordance with the reaction:

Various decarburizing atmospheres may be employed. These include pure hydrogen which, however, is expensive, although its use may sometimes be dictated by other factors including the development of magnetic properties :in magnetic ferrous sheet stocks. Dissociated ammonia is rich in hydrogen, but because it contains about 25% nitrogen, and by reason of its mode of manufacture, is less expensive than pure hydrogen. It may be made in such a way as to be substantially free of ammonia, so .as to have no nitriding effect. Other gases may be used, including mixtures of gases. For example a gas to which the designation HNX has been applied is a gas formed by the partial combustion of a fuel such as natural gas and treated, however, for the removal of carbon monoxide and carbon dioxide. This gas may be employed either alone or enriched with hydrogen or dissociated ammonia. The gases as introduced should be substantially free of carbon monoxide and carbon dioxide.

Reference may be made to the accompanying drawing which shows the relationship of carbon in the metal being treated to the percentage of carbon monoxide in the effluent decarburizing atmosphere. The carbon content of the metal is shown on the abscissa and the percentage of carbon monoxide on the ordinate. The curve was prepared for a ferrous material containing initially 044% carbon, which was decarburized in an atmosphere consisting of 40% hydrogen and 60% nitrogen, having a dew point of F., when all parts of the charge were at a steady temperature of 1300* F. The curve, however, is generally a plicable to any hydrogen bearing decarburizing atmosphere which as introduced does not contain carbonaceous gases. The curve shows that the percentage of carbon monoxide in the gas leaving the muffle is related to the percentage of carbon remaining in the charge.

Variations in temperature of the treatment and dew point of the annealing atmosphere will affect the shape of the curve. However, for each set of temperature and dew point conditions such a curve may be constructed so as to enable the operator to predict the residual carbon in the charge from the elfiuent gas analysis.

It is possible by varying the temperature and the dew point to slow down the decarburizing action in order to gain time in instances where a precise control of the residual carbon in the iron or steel is desired.

However, it will be usual to operate under standardized conditions including a temperature within the range set forth and a dew point of at least about 20 F. Higher dew points may be employed including those up to about 120 F. and higher.

It will be obvious that the internal capacity of the mufile, the volumetric flow of the atmosphere through it, and the surface area and gauge of the metal coil or coils being treated in the mufile can have an efiect upon the percentage of carbon monoxide in the effluent atmosphere. Nevertheless under conditions of good annealing practice where the factors of temperature, dew point and charge are substantially standardized, it is entirely possible within the scope of the invention to heat the coils uniformly to the chosen decarburization temperature, to thereafter introduce the chosen decarburizing atmosphere, and to stop the decarburization at the point where the carbon monoxide in the efliuent atmosphere indicates a desired residual carbon in the ferrous material being treated. The skilled worker in the art will further understand that where reference has been made herein to decarburizing and nondecarburizing atmospheres, within the scope of this inven tion, these atmospheres may be of entirely dissimilar compositions if desired for economy or other reasons, but that a non-decarburizing atmosphere may be converted quite easily to a decarburizing atmosphere by mixing it with a controlled quantity of steam. A decarburizing atmosphere may be quickly converted to a non-decarburizing atmosphere by shutting off the steam supply.

Example A thirty ton charge of ferrous strip having a gauge of .036 inch in open coil form was placed in an annealing mufile. During a heat-up cycle a dry, bright annealing atmosphere consisting of 40% H 60% N was maintained in the muffle. At the end of about twenty hours (when the thermocouples indicated that the ferrous material had been heated uniformly to a temperature of about 1300 F.) steam was admitted to the atmosphere to raise the dew point to 120 P. so as to start the decarburizing action. The eflluent gases from the muffle were subjected to gas analysis and the steam was turned off when the carbon monoxide content of the atmosphere reached a value of 10.5%. This value was attained in about one hour. The time involved will depend to a certain extent upon the gauge of the material and weight of the charge, being longer for heavier gauges and charge Weights.

The final product was found to be uniformly decarburized to a value of about .006%.

After the atmosphere has been changed from a decarburizing to a non-decarburizing condition, the soaking cycle may be prolonged as desired. Needless to say, the ferrous material will preferably be cooled in a suitable bright annealing atmosphere.

Modifications may be made in the invention without departing from the spirit of it.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the uniform partial decarburization of iron and steel strip having an initial carbon content in excess of .01%, the strip being in the form of loose coils located as a charge in a mufile furnace in which an annealing atmosphere is circulated with fresh atmosphere introduced into the furnace and spent atmosphere removed therefrom, the improvement comprising the steps of:

(a) heating said strip in a non-oxidizing atmosphere to a temperature in a range of from 1100 to 1600 F., the said strip being heated until all parts of the strip are at a uniform temperature within said range,

(b) introducing water vapor into said circulating atmosphere to raise the dew point ofsaid atmosphere to a value between 20 to F., said water vapor being introduced upon said strip reaching its said uniform temperature, thereby providing a decarburizing atmosphere which, when said strip is at said uniform temperature, will uniformly decarburize said strip,

(c) maintaining said decarburizing atmosphere until the strip has been partially decarburized to a specified intermediate carbon level, said specified intermediate carbon level being at least .005% and greater than the complete decarburization level for the particular strip material being treated,

(d) terminating the decarburization of the strip when said specified intermediate carbon level is reached by discontinuing the introduction of Water vapor into said atmosphere, and

(e) cooling the said partially decarburized strip in a non-oxidizing atmosphere.

2. The process claimed in claim 1 including the step of monitoring the carbon monoxide content of the spent atmosphere removed from said furnace to determine when the specified intermediate carbon level has been reached, the introduction of water vapor into the annealing atmosphere being discontinued when the carbon monoxide content of the spent atmosphere has reached a predetermined value above 1%.

3. The process claimed in claim 2 wherein the annealing atmosphere when introduced into the furnace is devoid of carburizing gases and contains suflicient hydrogen to prevent oxidation of iron at the temperatures involved.

References Cited UNITED STATES PATENTS CHARLES N, LOVELL, Primary Examiner. 

